Copolymers of acrylic acid and acrylamide (PAM) were introduced for improvement of soil quality via aggregation of soil particles over 50 years ago (U.S. Pat. Nos. 2,625,529 and 2,652,380 to Hedrick and Mowry (1953) and U.S. Pat. No. 2,652,381 to Basdekis (1953)). Commercial use of PAM in agriculture started to grow in the mid-90's and has gradually increased to levels of several million pounds per year of annual application in the U.S. In current use, the copolymers are generally dissolved in irrigation water at doses of 2 to 10 ppm, which converts to about 2 to 5 lbs per acre.
Principal benefits include soil retention and water conservation. Soil is retained on the fields via the agglomerating action of PAM as it flocculates soil constituents into larger, adhering particles that settle out of the flow. Hence, the water runs clear rather than turbid down the furrow. Moreover, because the fine powders and platelets of the soil become agglomerated as part of the settled particles, the natural porosity of the soil along the furrow does not become clogged with these fines as occurs in untreated furrows, and the infiltration of water into the soil is improved. Thus, water is conserved on the field, rather than flowing over and off the field, carrying topsoil with it. Ancillary benefits include reduced loss of adsorbed nutrients, fertilizers, and treatment chemicals such as herbicides and pesticides from the soil. These benefits lead to overall improvements in yields and crop quality.
On the other hand, there are problems with the ongoing usage of PAM in agriculture. For example, the polymers are generally very high MW (e.g. 12 to 22 million Da), which makes them slow to dissolve in water and produces very high viscosity solutions. Preparation of aqueous solutions requires vigorous stirring for extended periods to prevent the formation of viscous, gelatinous clumps, which remain insoluble and tend to clog metering equipment and delivery systems. Handling issues of this nature have significantly hindered the adoption of PAM by many growers.
In addition, the base monomer, acrylamide, is a hazardous, reactive monomer that is a known neurotoxin and suspected carcinogen. Of course, effective steps are taken to ensure that residual levels of monomer in the polymer products are well within safety limits. Nonetheless, it is evident that there are several properties and features of the copolymers of acrylate and acrylamide that are not desirable. Consequently, alternative materials are sought that will function well in agglomerating soil particles, without the handling issues and perceived environmental problems associated with PAM.